the standard is usually A1, though that isn't to say your work has to be A1 in size, but the composition of placing A4 and A3 images within such a space of white background is aesthetically pleasing, though it's entirely up to you. where i studied foundation, A1 was what was drilled into us and personally i think work looks great centered on that. just make sure your drop, window, slit, etc mounts are all measured up right n' tight, of course.

all of my work was mounted onto A1 card and then slipped inside a black A1 folder, but it wasn't ring bound or attached in anyway, instead the work was arranged loose so that it could be taken out for closer inspection, but it was sorted in a certain order all on top of each other. again, that was the standard from our foundation - there's nothing wrong with having it ring bound, but i'll warn you against plastic sleeves as it detracts from the work and if anything is textural/paint based, it looses alot of it's quality from being flattened by a horrible shiney skin.

as for where to buy, my dad got me mine from the works, though that was summer of 2008 and i don't even know if the works store is around anymore. it was pretty cheap, really if you were to be working the same method as what i said the folder itself isn't too important - black, simple, not too heavy or complicated. zip open, zip close.

I'd personally say A2 size (not too big, not too small...) but it depends what your currently working on. For example, most of the work that I do on my foundation course is A1 size, so this basically means that I *have* to have an A1 size portfolio.